The invention is in the field of shelter structures and is related particularly to structures using support elements and curved, lightweight roof membrane shapes spanning them. Examples are air supported structures, tension structures, tent-like structures, arch-supported membranes and free-span, point-supported systems. A survey of fabric tension structures for permanent buildings can be found in a presentation by the inventor herein to the International Symposium on Spatial Roof Structures at Dortmund, Germany, Sept. 10, 1984, entitled "A Decade Of Fabric Tension Structures For Permanent Buildings". Additional background material on such structures can be found in the twelve (12) references cited at pp. 19 and 20 of this presentation. The presentation and the 12 references are hereby incorporated by reference in this specification as though fully set forth herein.
Tension structures depend upon shape and prestress for their stability and capacity of carrying superimposed loads. Tent-like tension structures are generally center-supported systems which may use suspension bridge concepts and double cantilevers. One or several peaks supported by central masts are the common forms. The "Florida Festival" structures at Seaworld in Orlando, Fla., use the tent principle in a composition of vertical tents and their inversion. The roof membranes are supported by the radial cables spanning from central poles to concrete edge beams. When internal supports are not desirable special means have to be developed to achieve free spans. Arch supported structures are one method of developing free spans. One example is the Bullocks Department Store in San Jose, Calif., in which the roof fabric rides over a system of arches. Another method of creating free-span structures relies on point-supported systems; for example systems in which the free span is created by a large A-frame supporting the main structural peak, from overhead, in combination with peripheral posts restrained by stay cables. An example is the outdoor pavilion for Crown Center in Kansas City, Mo. Other examples of free span tension structures are described or referred to in said presentation and its references.
The desirable characteristics of structures of this general type include high strength to weight ratio, ease of erection and low cost. While much progress has been made in this field, it is believed that much need still remains for achieving a significantly improved balance of these and other desirable characteristics, and the invention is directed to meeting that need.
One principle utilized in this invention is the use of double curvature to create a stable form, to allow an economical stress flow and to minimize the use of compression members except directly over the support members.
In a particular and nonlimiting embodiment, the invention comprises a saddle-shaped cable dome system for large-span, lightweight roof structures, and uses the curvature of a saddle surface, combined with two orthogonal cable nets separated by a set of compression struts, to create an efficient structural system confined by an edge ring loaded primarily in compression. The edge ring can take on a circular, elliptic, superelliptic, or approximately superelliptic shape in plan view. The two saddle surface nets are generated by one direction of each net taking on a very small curvature and the other direction assuming a significantly greater curvature. The elements and the interaction thereof are selected such that the result is a funnicular edge ring under pre-stress. The compression struts connect related node points of the two nets, transferring loads between them. As a result, the cables with the greater curvatures are the primary carriers of loads, while the cables with the shallower curvature act primarily as restraining cables. If the supports of the edge ring are allowed to move without horizontal restraint, the system can adjust to find equilibrium even if the edge ring is hinged at the node points. Support restraints and ring stiffness reduce the amount of ring movement required to achieve equilibrium.
The curvature of the upper carrying cables which support the roof membrane results in the double curved configuration of the membrane. In the direction of the upper carrying cables the membrane takes on a curvature parallel to that of the upper carrying cables. At right angles to the upper carrying cables the membrane takes on curvature in the opposite direction. As a result, it sags down between the cables giving it the correct shape to carry downward loads, such as snow. Under the wind suction loading, the membrane is pulled upwards until it reverses its curvature at right angles to the upper carrying cables (see dashed line of FIG. 3). This is easily accommodated because the curvature in the long direction is very small. Therefore, the increase in stress in the long direction is acceptable. It has the added advantage of absorbing part of the wind load. By this method, the membrane is capable of carrying any combination of anticipated loads.
The exemplary system allows for particularly efficient construction. For example, after construction of the edge ring and its supports, the main carrying cables for downward loads of the lower net are installed, then the restraining cables of the same net are placed on top of the carrying cables to form the lower net. Then the restraining cables of the upper net are placed over the lower net. Next, the compression struts are placed on the lower net nodes and held in place near their upper ends by the upper restraint cables. The upper carrying cables are then installed, completing the upper net. Finally, the net systems are prestressed, completing the primary structural system. Prestressing can be achieved by tensioning of cables, expanding of struts, or a combination of these two methods.
In the case of using a fabric roof, the compression struts can extend above the level of the upper restraining cables by the distance required for the fabric curvature. The upper carrying cables can be placed on top of the strut extensions, with the fabric skin attached to the upper carrying cables and forming trough-like long strips which sag between adjacent carrying cables. An inner liner can be secured to the upper or the lower net. The shape of the substantially parallel, trough-like strips of outer skin allows for the use of a retractable center portion of the roof.
More specifically, an exemplary embodiment of the invention comprises a cable dome system using a substantially rigid, generally laterally extending edge ring which is loaded primarily in compression. A lower cable net is secured to the edge ring and comprises a set of carrying cables intersected by a set of restraining cables running in a direction transverse to that of the carrying cables, to form therewith a substantially rectangular (in plan view) lower grid. An upper cable net is secured to the edge ring and comprises a set of carrying cables aligned (in plan view) with the restraining cables of the lower net and a set of restraining cables aligned (in plan view) with the carrying cables of the lower net, to form a similar substantially rectangular (in plan view), upper grid. The cables of the lower net intersect at an array of lower nodes, and those of the upper net intersect at a similar array of upper nodes which are generally aligned (in plan view) with those of the lower net. Upwardly extending compression struts secure to each other, and space from each other, the vertically aligned lower and upper nodes. The curvatures of the carrying cables (in elevational view) are significantly greater than those of the restraining cables, so that the carrying cables act as the primary carriers of vertical loads as compared with the restraining cables. The curvatures of the carrying cables of the upper are convex in elevational view, and the curvatures of the carrying cables of the lower net are concave in elevational view. The carrying cables of the upper net can be above and spaced from the restraining cables of the same net, so that a membrane secured to the upper carrying cables can sag and form troughs between the upper carrying cables.
Because the trough-like runs of the roof skin have substantially parallel sides, simple, rectangular strips of roofing material can be installed easily and can maintain high strength. Little fabric waste results, because the only nonrectangular skin shapes are those at the edge ring. The upper carrying cables can be uniformly spaced from each other, to allow for constant width roof strips which, when installed, form convenient troughs for water runoff. In addition, the constant width of the skin strips between the upper carrying cables allows for the convenient use of systems for retracting central portions of the roof. The edge ring can be unitary (for example, it can be made of continuous reinforced concrete or of welded steel beams) or it can be assembled from individual sections.
The upper carrying cables are so located that the distance between them is constant in any one bay. This makes it possible to install a retractable roof with tracks loaded on top of the upper carrying cables.